Cellulose acetate has historically been used as a support material in the manufacture of photographic films. In particular, cellulose acetate films are known to have many desirable properties necessary for a photographic film base including high transparency and relatively good dimensional stability with respect to moisture and temperature. Cellulose acetate also has excellent resistance to degradation by ultraviolet light and does not readily discolor when exposed to light or heat. These attributes have also made cellulose acetate films useful for the fabrication of optical devices. For example, light polarizer plates utilize cellulose acetate films as covers to protect the delicate polarizing film from distortion and damage. In this regard, cellulose acetate films have replaced glass to produce lightweight, flexible polarizer plates. Polarizers having cellulose acetate protective covers are utilized in liquid crystal displays, OLED (organic light emitting diode) displays, and in other electronic displays found in, for example, personal computers, televisions, cell phones, and instrument panels.
Polymers of the cellulose acetate type are commercially available in a variety of molecular weights as well as the degree of acyl substitution of the hyroxyl groups on the cellulose backbone. Of these, the fully substituted polymer, cellulose triacetate (CTA) having a combined acetic acid content of approximately 60% is commonly used to manufacture cellulose acetate films.
In general, resin films are prepared either by melt extrusion methods or by casting methods. Melt extrusion methods involve heating the resin until molten (approximate viscosity on the order of 100,000 cp), and then applying the hot molten polymer to a highly polished metal band or drum with an extrusion die, cooling the film, and finally peeling the film from the metal support. For many reasons, however, films prepared by melt extrusion are generally not suitable for optical applications. Principal among these is the fact that melt extruded films exhibit a high degree of optical birefringence. In the case of highly substituted cellulose acetate, there is the additional problem of melting the polymer. Cellulose triacetate has a very high melting temperature of 270-300° C., and this is above the temperature where decomposition begins. Films have been formed by melt extrusion at lower temperatures by compounding cellulose acetate with various plasticizers as taught in U.S. Pat. No. 5,219,510 to Machell. However, the polymers described in U.S. Pat. No. 5,219,510 to Machell are not the fully substituted cellulose triacetate, but rather have a lesser degree of alkyl substitution or have proprionate groups in place of acetate groups. Even so, melt extruded films of cellulose acetate are known to exhibit poor flatness as noted in U.S. Pat. No. 5,753,140 to Shigenmura. For these reasons, melt extrusion methods are generally not practical for fabricating many resin films including cellulose triacetate films used to prepare protective covers and substrates in electronic displays. Rather, casting methods are generally used to manufacture these films.
Resin films for optical applications are manufactured almost exclusively by casting methods. Casting methods involve first dissolving the polymer in an appropriate solvent to form a dope having a high viscosity on the order of 50,000 cp, and then applying the viscous dope to a continuous highly polished metal band or drum through an extrusion die, partially drying the wet film, peeling the partially dried film from the metal support, and conveying the partially dried film through an oven to more completely remove solvent from the film. Cast films typically have a final dry thickness in the range of 40-200 microns. In general, thin films of less than 40 microns are very difficult to produce by casting methods due to the fragility of wet film during the peeling and drying processes. Films having a thickness of greater than 200 microns are also problematic to manufacture due to difficulties associated with the removal of solvent in the final drying step. Although the dissolution and drying steps of the casting method add complexity and expense, cast films generally have better optical properties when compared to films prepared by melt extrusion methods, and problems associated with decomposition at high temperature are avoided.
Examples of optical films prepared by casting methods include: 1.) Cellulose acetate sheets used to prepare light polarizers as disclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289 to Cael as well as more recent disclosures in U.S. Patent Application. Serial no. 2001/0039319 A1 to Harita and U.S. Patent Application. Serial no. 2002/001700 A1 to Sanefuji, 2.) Cellulose triacetate sheets used for protective covers for light polarizers as disclosed in U.S. Pat. No. 5,695,694 to Iwata, 3.) Polycarbonate sheets used for protective covers for light polarizers or for retardation plates as disclosed in U.S. Pat. No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani, and 4.) Polyethersulfone sheets used for protective covers for light polarizers or for retardation plates as disclosed in U.S. Pat. Nos. 5,759,449 and 5,958,305 both to Shiro.
Despite the wide use of the casting method to manufacture optical films, there are however, a number of disadvantages to casting technology. One disadvantage is that cast films have significant optical birefringence. Although films prepared by casting methods have lower birefringence when compared to films prepared by melt extrusion methods, birefringence remains objectionably high. For example, cellulose triacetate films prepared by casting methods exhibit in-plane retardation of 7 nanometers (nm) for light in the visible spectrum as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Polycarbonate films prepared by casting methods exhibit in-plane retardation of 17 nm as disclosed in U.S. Pat. Nos. 5,478,518 and 5,561,180 both to Taketani. U.S. Patent Application. Serial No. 2001/0039319 A1 to Harita claims that color irregularities in stretched cellulose acetate sheets are reduced when the difference in retardation between widthwise positions within the film is less than 5 nm in the original unstretched film. For many applications of optical films, low in-plane retardation values are desirable. In particular, values of in-plane retardation of less than 10 nm are preferred.
Birefringence in cast films arises from orientation of polymers during the manufacturing operations. This molecular orientation causes indices of refraction within the plane of the film to be measurably different. In-plane birefringence is the difference between these indices of refraction in perpendicular directions within the plane of the film. The absolute value of birefringence multiplied by the film thickness is defined as in-plane retardation. Therefore, in-plane retardation is a measure of molecular anisotropy within the plane of the film.
During the casting process, molecular orientation may arise from a number of sources including shear of the dope in the die, shear of the dope by the metal support during application, shear of the partially dried film during the peeling step, and shear of the free-standing film during conveyance through the final drying step. These shear forces orient the polymer molecules and ultimately give rise to undesirably high birefringence or retardation values. To minimize shear and obtain the lowest birefringence films, casting processes are typically operated at very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Slower line speeds generally produce the highest quality films.
Another drawback to the casting method is the inability to accurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 to Hayward, conventional multi-slot casting dies create unacceptably non-uniform films. In particular, line and streak non-uniformity is greater than 5% with prior art devices. Acceptable two layer films may be prepared by employing special die lip designs as taught in U.S. Pat. No. 5,256,357 to Hayward, but the die designs are complex and may be impractical for applying more than two layers simultaneously.
Another drawback to the casting method is the restrictions on the viscosity of the dope. In casting practice, the viscosity of dope is on the order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Hayward describes practical casting examples using dopes with a viscosity of 100,000 cp. In general, cast films prepared with lower viscosity dopes are known to produce non-uniform films as noted for example in U.S. Pat. No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowest viscosity dopes used to prepare casting samples are approximately 10,000 cp. At these high viscosity values, however, casting dopes are difficult to filter and degas. While fibers and larger debris may be removed, softer materials such as polymer slugs are more difficult to filter at the high pressures found in dope delivery systems. Particulate and bubble artifacts create conspicuous inclusion defects as well as streaks and may create substantial waste.
In addition, the casting method can be relatively inflexible with respect to product changes. Because casting requires high viscosity dopes, changing product formulations requires extensive down time for cleaning delivery systems to eliminate the possibility of contamination. Particularly problematic are formulation changes involving incompatible polymers and solvents. In fact, formulation changes are so time consuming and expensive with the casting method that most production machines are dedicated exclusively to producing only one film type.
Finally, cast films may exhibit undesirable cockle or wrinkles. Thinner films are especially vulnerable to dimensional artifacts either during the peeling and drying steps of the casting process or during subsequent handling of the film. In particular, the preparation of composite optical plates from resin films requires a lamination process involving application of adhesives, pressure, and high temperatures. Very thin films are difficult to handle during this lamination process without wrinkling. In addition, many cast films may naturally become distorted over time due to the effects of moisture. For optical films, good dimensional stability is necessary during storage as well as during subsequent fabrication of composite optical plates.